Estimating the location of inexpensive wireless terminals by using signal strength measurements

ABSTRACT

A system is disclosed that enables the estimation of the location of a wireless terminal in a wireless network. The illustrative embodiment works without requiring modifications to be made to the wireless terminal. Furthermore, the hardware of some embodiments of the present invention can be inexpensively deployed indoors. Some embodiments of the present invention are, therefore, ideally suited for use with legacy indoor systems. The system of the illustrative embodiment of the present invention, in some embodiments, uses an offline process and an online process for location estimation. The described system, however, can be used with other techniques for location estimation.

FIELD OF THE INVENTION

The present invention relates to telecommunications in general, and,more particularly, to estimating the location of a wireless terminal.

BACKGROUND OF THE INVENTION

FIG. 1 depicts a schematic diagram of wireless network 100 in the priorart, which comprises wireless telecommunication terminal 101 and accesspoints 102-1 through 102-L, interconnected as shown. Wireless terminal101 uses access points 102-1 through 102-L to exchange blocks of data,or “packets,” with computer servers that are external to wirelessnetwork 100. At any given time, wireless terminal 101 is associated withone of access points 102-1 through 102-L for the purpose ofcommunicating with the external servers.

It is important in some operating scenarios to know the location ofwireless terminal 101 within wireless network 100. Knowledge of thelocation of wireless terminal 101 enables services that use end-userlocation information. Such services include location-aware contentdelivery, emergency location, services based on the notion of “closestresource,” and location-based access control.

Various techniques for estimating location exist in the prior art.

In accordance with one technique, the location of a wireless terminal isestimated to be at the center of the coverage area served by the accesspoint with which the wireless terminal is associated. This technique isadvantageous in that it does not require that additional hardware beadded to the wireless terminal or to the wireless network, and thismeans that the first technique can be inexpensively implemented inlegacy systems. The accuracy of the first technique, however, is afunction of the size of the coverage area, and, therefore, it isgenerally not acceptable for applications that require higher accuracy.

In accordance with a second technique, the location of a wirelessterminal is estimated by a radio navigation unit, such as a GlobalPositioning System receiver, that is incorporated into the wirelessterminal. This technique is accurate to within a few meters and isadvantageous in that it does not require that additional hardware beadded to the telecommunication system's infrastructure. The secondtechnique is disadvantageous, however, in that it cannot be used withlegacy wireless terminals that do not comprise a radio navigation unitand, furthermore, cannot be used reliably indoors.

In accordance with a third technique, the location of a wirelessterminal is estimated by triangulating the angle of arrival or the timeof arrival of the signals transmitted by the wireless terminal to belocated. Various receivers that are either part of or separate from theaccess points are part of the triangulation process. This technique isaccurate to within a few meters and is advantageous in that it can beused with legacy wireless terminals. It is disadvantageous, however, inthat it generally requires that specialized hardware be added to thewireless network, and this is very expensive.

In accordance with a fourth technique, the location of a wirelessterminal is estimated by using wireless terminals to make signalmeasurements and to transmit those measurements to another device foranalysis. This technique is accurate to within a few meters and isadvantageous in that it exploits a potentially large installed base ofwireless terminals. It is disadvantageous, however, in that it cannot beused with some legacy terminals. Such legacy terminals include thosethat do not have the ability to transmit signal strength measurementsand those that do not have the ability to maintain a signal strengthmeasurement database against which to match received signals.

Therefore, the need exists for a technique for estimating the locationof a wireless terminal with higher resolution than the first techniqueand that can be inexpensively implemented in legacy systems.

SUMMARY OF THE INVENTION

The present invention enables the estimation of the location of awireless terminal in a wireless network without some of the costs anddisadvantages associated with techniques in the prior art. For example,some embodiments of the present invention operate without anymodifications to the wireless terminal. Furthermore, some embodiments ofthe present invention can be deployed indoors. And still furthermore,some embodiments of the present invention are inexpensive andwell-suited for use with wireless terminals and base stations that arenot capable of measuring the strength of the signals emitted by thewireless terminals.

In some embodiments of the present invention, a plurality oftransmit-only devices, hereinafter called “emitters,” transmit one ormore signals for the purpose of providing reference signals to measure.The emitters can be small, battery-operated, easy-to-situate devicesthat occasionally transmit a signal (representing a packet of data, forexample). Each emitter is deployed at a known location.

A plurality of receivers, hereinafter called “signal monitors,” measurethe signal strength of the signals transmitted by each of the emitters,and each signal monitor is able to differentiate which emitterstransmitted which signals. Each signal monitor is deployed at a knownlocation.

Because the emitters and the signal monitors are deployed at knownlocations, and because the signal monitors can distinguish which signalsare transmitted by which emitters, the collection of signal strengthmeasurements can be used to build a database that correlatessignal-strength measurements to location.

When the location of a wireless terminal is desired, the signal monitorscan measure the signal strength of the signals from that wirelessterminal, and using the database, estimate the location of the wirelessterminal based on the signal-strength measurements.

Some embodiments of the present invention use both an offline processand an online process for location estimation. It will be clear,however, to those skilled in the art, after reading this specification,how to make and use the system with another process for locationestimation.

For example, some embodiments use an offline model-building procedureand then an online estimation procedure. In the offline procedure, thesignal monitors provide the signal strength measurements to a locationestimation server. For each signal monitor's set of signal strengthmeasurements, the location estimation server performs a two-dimensionalinterpolation on the measurements acquired from the signal monitor togenerate signal strength samples throughout a grid of points thatcorresponds to the wireless network. The interpolation, in someembodiments, is performed using Akima splines. The location estimationserver then combines the samples to get signal strength vectors for eachgrid point, which results in a synthetic model.

In the online estimation part, the location estimation server matches apresented signal strength vector against the synthetic model todetermine the closest match. The closest matching grid point isidentified as the location of the wireless terminal.

An illustrative embodiment of the present invention comprises: a firstsignal monitor for measuring: (i) the signal strength at the firstsignal monitor of a first packet transmitted by an emitter; and (ii) thesignal strength at the first signal monitor of a second packettransmitted by a wireless terminal; and a processor for: (a) receivingthe location of the emitter; and (b) determining the location of thewireless terminal based on (i), (ii), and the location of the emitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of wireless network 100 in the priorart.

FIG. 2 depicts a schematic diagram of network 200 in accordance with theillustrative embodiment of the present invention.

FIG. 3 depicts a block diagram of the salient components of emitter201-i in accordance with the illustrative embodiment of the presentinvention.

FIG. 4 depicts a block diagram of the salient components of signalmonitor 202-j in accordance with the illustrative embodiment of thepresent invention.

FIG. 5 depicts a block diagram of the salient components of locationestimation server 203 in accordance with the illustrative embodiment ofthe present invention.

FIG. 6 depicts a flowchart that summarizes the tasks performed by theillustrative embodiment of the present invention.

FIG. 7 depicts a flowchart of task 601 performed in accordance with theillustrative embodiment of the present invention.

FIG. 8 depicts a flowchart of task 602 performed in accordance with theillustrative embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 2 depicts a schematic diagram of network 200 that comprises thecomponents of the illustrative embodiment of the present invention.Network 200 operates in accordance with a set of air interface protocols(e.g., IEEE 802.11, etc.) and comprises signal emitters 201-1 through201-M, wherein M is a positive integer; signal monitors 202-1 through202-N, wherein N is a positive integer; location estimation server 203;wireless terminal 204; and access point 205, interconnected as shown.

Signal emitter 201-i, for i=1 to M, is capable of transmitting a signalover a wireless medium in well-known fashion. The signal can representpackets of data that comprise information used for identifying emitter201-i. Emitter 201-i is stationary in some embodiments. In someembodiments, the identifying information indicates the location ofemitter 201-i. In other embodiments, the identifying information is themedium access control address of emitter 201-i. The salient details ofemitter 201-i are described below and with respect to FIG. 3.

Signal monitor 202-j, for j=1 to N, measures (i.e., “sniffs”) signalsthat are present on the wireless medium and transmitted by varioussignal sources, and determines the received signal strength (RSS) ofthose signals. Signal sources include emitter 201-i and wirelessterminal 204. Signal monitor 202-j sends the signal strengthmeasurements to location estimation server 203. In addition, in someembodiments signal monitor 202-j receives the identifying informationtransmitted by emitter 201-i or wireless terminal 204 or both and sendsthe information to location estimation server 203. In some embodiments,signal monitor 202-j provides information (e.g., its coordinates, itsidentifier, etc.) with which to determine its location-either directlyor indirectly—to location estimation server 203. The salient details ofsignal monitor 202-j are described below and with respect to FIG. 4.

FIG. 2 depicts a wired interface between signal monitors 202-1 through202-N and location estimation server 203. Signal monitors 202-1 through202-N, however, can communicate with location estimation server 203 viaa wired interface, the wireless medium, or both in well-known fashion.

Location estimation server 203 acquires the received signal strengthmeasurements from signal monitors 202-1 through 202-N. Locationestimation server processes the received signal strength measurementscorresponding to one or more of emitters 201-1 through 201-M inaccordance with the illustrative embodiment of the present invention.The salient details of location estimation server 203 are describedbelow and with respect to FIG. 5.

Wireless terminal 204 is capable of transmitting packets of data over awireless medium in well-known fashion. The packets of data can compriseinformation that identifies wireless terminal 204. Wireless terminal 204comprises a transmitter for the purpose of transmitting the packets ofdata. Wireless terminal 204 can be a communications station, a locatingdevice, a handheld computer, a laptop with wireless capability, atelephone, etc. It will be clear to those skilled in the art how to makeand use wireless terminal 204.

Wireless terminal 204, in some embodiments, exchanges packets withaccess point 205. Signal monitor 202-j can measure these packets for thepurpose of estimating location. In other embodiments, wireless terminal204 transmits packets specifically for the purpose of estimating thelocation of wireless terminal 204.

Access point 205, in some embodiments, exchanges packets of data withwireless terminal 204 in well-known fashion. Access point 205 can beused to coordinate communication in network 200 and to provide wirelessterminal 204 with access to networks that are external to network 200,in well-known fashion. In other embodiments, access point 205 is notpresent. It will be clear to those skilled in the art how to make anduse access point 205.

Guidelines on where to place emitter 201-i within the area served bywireless network 200 are described here. The area served by wirelessnetwork 200 is divided into sub-regions of approximately equal size suchthat at least seven sub-regions are within transmitting range of each ofsignal monitors 202-1 through 202-N. The seven sub-regions allow forreasonable smoothing, a technique that is described later. Each emitter201-i is then placed as closely as possible to the center of thecorresponding sub-region. If smoothing is done using a generalizedadditive model, as is known in the art, it is beneficial to placeemitters 201-1 through 201-M to present a large number of distinct x andy coordinates of a grid that represents the area served, while keepingemitters 201-1 through 201-M as far apart as possible. The gridmentioned here is described later. It will be clear to those skilled inthe art, after reading this specification, that other techniques can beused to place emitters 201-1 through 201-M.

Guidelines on where to place signal monitor 202-j are described here. Insome embodiments, signal monitor 202-j and access point 205 arecollocated. In other embodiments, additional signal monitors, or signalmonitors not collocated with access point 205 are placed to ensure thatsignal monitors 202-1 through 202-N are not collinear (or no threesignal monitors are collinear) within the x-y coordinate plane mentionedearlier.

FIG. 3 depicts a block diagram of the salient components of emitter201-i in accordance with the illustrative embodiment of the presentinvention. Emitter 201-i comprises transmitter 301, processor 302, andmemory 303, interconnected as shown. Emitter 201-i can bebattery-operated or powered via an external source.

Transmitter 301 is a circuit that is capable of transmitting packetsinto the wireless medium, in well-known fashion, from processor 302. Itwill be clear to those skilled in the art how to make and usetransmitter 301.

Processor 302 is a general-purpose processor that is capable ofperforming the tasks described below and with respect to FIGS. 6 and 7.It will be clear to those skilled in the art, after reading thisspecification, how to make and use processor 302.

Memory 303 is capable of storing programs and data used by processor302. It will be clear to those skilled in the art how to make and usememory 303.

FIG. 4 depicts a block diagram of the salient components of signalmonitor 202-j in accordance with the illustrative embodiment of thepresent invention. Signal monitor 202-j comprises receiver 401,processor 402, and memory 403, interconnected as shown.

Receiver 401 is a circuit that is capable of receiving packets from thewireless medium, in well-known fashion, and of forwarding them toprocessor 402. It will be clear to those skilled in the art how to makeand use receiver 401.

Processor 402 is a general-purpose processor that is capable ofperforming the tasks described below and with respect to FIGS. 6 through8. It will be clear to those skilled in the art, after reading thisspecification, how to make and use processor 402.

Memory 403 is capable of storing programs and data used by processor402. It will be clear to those skilled in the art how to make and usememory 403.

FIG. 5 depicts a block diagram of the salient components of locationestimation server 203 in accordance with the illustrative embodiment ofthe present invention. Location estimation server 203 comprises networkinterface 501, processor 502, and memory 503, interconnected as shown.

Network interface 501 is a circuit that is capable of receiving, inwell-known fashion, received signal strength measurements and emitteridentifier information from one or more of signal monitors 202-1 through202-N. In some embodiments, network interface 501 receives signalmonitor identifier information from one or more of signal monitors 202-1through 202-N. Network interface 501 is also capable of forwarding thesignal strength measurements and identifier information received toprocessor 502. It will be clear to those skilled in the art how to makeand use network interface 501.

Processor 502 is a general-purpose processor that is capable ofperforming the tasks described below and with respect to FIGS. 6 through8. It will be clear to those skilled in the art, after reading thisspecification, how to make and use processor 502.

Memory 503 is capable of storing programs and data used by processor502. It will be clear to those skilled in the art how to make and usememory 503.

FIG. 6 depicts an overview of a location estimation technique that canbe performed by the system of the illustrative embodiment. The techniqueuses an offline process to build a signal strength model, followed by anonline process to compare a presented signal strength vector against thesignal strength model built. It will be clear, however, to those skilledin the art how to apply a different technique to the system describedwith respect to FIGS. 2 through 5.

At task 601 as part of the offline process, location estimation server203 acquires from one or more of signal monitors 202-1 through 202-N thereceived signal strength measurements from one or more of emitters 201-1through 201-M. Location estimation server 203 also acquires theidentifiers (e.g., medium access control addresses, locationcoordinates, etc.) of one or more of emitters 201-1 through 201-M andmaps the identifiers to emitter location. Location estimation server 203uses the acquired information to build-and refine, if needed-its signalstrength model for location estimation.

At task 602 as part of the online process, when wireless terminal 204needs to be located or tracked, location estimation server 203 uses thesignal strength measurements corresponding to wireless terminal 204, asmade by signal monitors 202-1 through 202-N, in conjunction with thesignal strength model built.

FIG. 7 depicts a flowchart of the salient tasks that constitute task601, performed in accordance with the illustrative embodiment of thepresent invention. It will be clear to those skilled in the art whichtasks depicted in FIG. 7 can be performed simultaneously or in adifferent order than that depicted.

At task 701, each of emitters 201-1 through 201-M transmits one or morepackets over the wireless medium. Emitter 201-i transmits the packetsperiodically or sporadically. In some embodiments, emitter 201-i isprompted by another device (e.g., signal monitor 202-j, etc.) totransmit a packet. Typically, emitter 201-i only has to transmit thepackets occasionally (e.g., a couple of times each hour, etc.),depending on how fast the signal strength model needs to be verified andupdated. In some embodiments, emitter 201-i transmits an identifier.

At task 702, each of signal monitors 202-1 through 202-N measures thesignal strength of each packet.

At task 703, location estimation server 203 acquires a plurality ofsignal strength measurements from each of signal monitors 202-1 through202-N. The location of each emitter 201-i is also made available to ordetermined by location estimation server 203. For instance, locationestimation server 203 can receive the coordinates of emitter 201-i.Alternatively, location estimation server 203 can receive the mediumaccess control address of emitter 201-i and derive emitter 201-i'slocation from a lookup table.

In some embodiments, when signal strength measurements are missing forone or more signal emitters, location estimation server 203 assignssignal strength measurement values that are pegged low (e.g., −92 dBm,etc.) to represent those emitters for which signal strength measurementshave not been received. Pegging the emitter's missing reading to a lowvalue, in essence, indicates that the point in question is far away fromthe signal monitor 202-j.

At this point, location estimation server 203 receives a signal strengthmeasurement (i.e., actual or pegged values) that is representative ofeach of emitters 201-1 through 201-M from each of signal monitors 202-1through 202-N. Furthermore, location estimation server 203 has availablethe emitter location from which each signal strength measurement wastransmitted.

In some embodiments, the signal strength measurement that represents aparticular signal source is actually either (i) the median of or (ii)the mean of more than one signal strength measurement made over time onmultiple packets transmitted by that signal source. It will be clear tothose skilled in the art how to determine either the median or the meanof more than one signal strength measurement.

At task 704, in some embodiments, location estimation server 203smoothes, in well-known fashion, each signal monitor's plurality ofsignal strength measurements received from one or more of signalmonitors 202-1 through 202-N. An example of a smoothing algorithm uses ageneralized additive model (GAM), as is known in the art. It will beclear to those skilled in the art when to apply smoothing and when notto apply smoothing. For example, when there are few signal emitters(i.e., the value M is small) that are far apart, smoothing becomes lessimportant.

Location estimation server 203 then generates a synthetic model by firstdividing the wireless network area being modeled into grid squares ofknown size (e.g., three feet by three feet, etc.). At task 705 locationestimation server 203 interpolates over two dimensions, for each signalmonitor independently, each plurality of signal strength measurements,already smoothed in some embodiments, to estimate the received signalstrength at the center of each grid square. In some embodiments,location estimation server 203 uses Akima splines for the interpolation.Akima spline interpolation is a local, triangle-based technique, as isknown in the art. The resultant synthetic model for each signal monitor202-j is a scalar array of signal strengths with an estimated signalstrength that corresponds to each grid square. Location estimate server203 uses the known locations of emitters 201-1 through 201-M and thereceived signal strengths that correspond to signals transmitted fromthose known locations to compute the model in well-known fashion.

At task 706, after processing data from signal monitors 202-1 through202-N, location estimation server 203 generates a vector array bycombining the scalar arrays formed at task 705. If scalar arrays for allof signal monitors 202-1 through 202-N are available, each vector arraycell, which corresponds to grid square, has an associated N-vector ofsignal strengths.

Location estimation server 203 repeats tasks 701 through 706 wheneverthe model needs to be rebuilt. For instance, location estimation server203 can rebuild the model when at least one of signal monitors 202-1through 202-N measures a signal strength from any of emitters 201-1through 201-M that consistently exhibits a statistically significantdeviation in the ongoing course of transmitting signals. It will beclear to those skilled in the art when to rebuild the model.

FIG. 8 depicts the salient tasks that constitute task 602, performed inaccordance with the illustrative embodiment of the present invention.

At task 801, signal monitor 202-j measures the signal strength of atleast one packet transmitted by wireless terminal 204. Note that thelocation of the wireless terminal is unknown before measuring the signalstrength of the packet. Furthermore, no previous information on thewireless terminal is available necessarily to the system of theillustrative embodiment. The signal strength measurement of the packettransmitted by wireless terminal 204, along with the signal strengthmeasurement of the packet and the identifier transmitted by each emitter201-i, is used to determine the location of wireless terminal 204.

In some embodiments, the signal strength measurement that representswireless terminal 204 is actually either (i) the median of or (ii) themean of more than one signal strength measurement made over time onmultiple packets transmitted by wireless terminal 204. It will be clearto those skilled in the art how to determine either the median or themean of more than one signal strength measurement.

In some embodiments, wireless terminal 204 is prompted by another device(e.g., access point 205, etc.) to transmit a packet.

At task 802, location estimation server 203 receives one or more signalstrength measurements of wireless terminal 204 from at least one ofsignal monitors 202-1 through 202-N. Location estimation server 203combines the measurements to form a presented signal strength vector inwell-known fashion.

Location estimation server 203, in some embodiments, assigns a valuepegged to a low number (e.g., −92 dBm, etc.) in place of signals notreceived from one or more signal monitors, in order to infer location.

At task 803, location estimation server 203 matches the presented signalstrength vector as detected by signal monitors 202-1 through 202-N withthe synthetic model to locate the wireless terminal. In someembodiments, location estimation server 203 uses a nearest neighborsearch (NNS), as is known in the art. It will be clear to those skilledin the art how to match a presented vector with an array of syntheticmodel vectors.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. For example, in thisSpecification, numerous specific details are provided in order provide athorough description and understanding of the illustrative embodimentsof the present invention. Those skilled in the art will recognize,however, that the invention can be practiced without one or more ofthose details, or with other methods, materials, components, etc.

Furthermore, in some instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the illustrative embodiments. It is understood that thevarious embodiments shown in the Figures are illustrative, and are notnecessarily drawn to scale. Reference throughout the specification to“one embodiment” or “an embodiment” or “some embodiments” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment(s) is included in at least one embodimentof the present invention, but not necessarily all embodiments.Consequently, the appearances of the phrase “in one embodiment,” “in anembodiment,” or “in some embodiments” in various places throughout theSpecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, materials, orcharacteristics can be combined in any suitable manner in one or moreembodiments. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

1. An apparatus comprising: a first signal monitor for measuring: (i)the signal strength at said first signal monitor of a first packettransmitted by an emitter; and (ii) the signal strength at said firstsignal monitor of a second packet transmitted by a wireless terminal;and a processor for: (a) receiving the location of said emitter; and (b)determining the location of said wireless terminal based on (i), (ii),and the location of said emitter.
 2. The apparatus of claim 1 furthercomprising said emitter for transmitting over a wireless medium saidfirst packet wherein said first packet comprises information related tothe location of said emitter.
 3. The apparatus of claim 1 furthercomprising a second signal monitor for measuring: (i) the signalstrength at said second signal monitor of said first packet transmittedby said emitter; and (ii) the signal strength at said second signalmonitor of said second packet transmitted by said wireless terminal. 4.The apparatus of claim 3 wherein said processor is also for: acquiring(i) a first plurality of signal strength measurements made by said firstsignal monitor and (ii) a second plurality of signal strengthmeasurements made by said second signal monitor, wherein said firstplurality and said second plurality each are associated with a pluralityof signal sources that comprise said emitter; interpolating (i) saidfirst plurality across two dimensions to form a first scalar array ofsignal samples and (ii) said second plurality across two dimensions toform a second scalar array of signal samples; and generating a vectorarray of synthetic signal strength vectors based on said first scalararray of signal samples and said second scalar array of signal samples.5. The apparatus of claim 4 wherein said processor is also for matchinga presented signal strength vector to at least one of said syntheticsignal strength vectors.
 6. The apparatus of claim 5 wherein matchingsaid presented signal strength vector comprises finding the closestmatch in signal vector space.
 7. The apparatus of claim 4 whereininterpolating said first plurality is performed using Akima splines. 8.The apparatus of claim 4 wherein said processor is also for smoothingsaid first plurality.
 9. The apparatus of claim 8 wherein smoothing isbased on a generalized additive model.
 10. The apparatus of claim 4wherein said processor is also for assigning a signal value in place ofa missing signal strength measurement.
 11. The apparatus of claim 10wherein said signal value is less than or equal to −92 dBm.
 12. Theapparatus of claim 4 wherein the signal strength measurement thatrepresents a signal source is one of (i) the median of and (ii) the meanof more than one signal strength measurement made over time of saidsignal source.
 13. The apparatus of claim 2 wherein said emitter isstationary.
 14. The apparatus of claim 2 wherein said informationrelated to the location of said emitter indicates the medium accesscontrol address of said emitter.
 15. The apparatus of claim 2 whereinsaid information related to the location of said emitter indicates thecoordinates of said emitter.
 16. An apparatus comprising: an emitter fortransmitting a signal and an identifier; a first signal monitor formaking: (i) a first plurality of signal strength measurements of saidsignal, wherein said first plurality is associated with a plurality ofsignal sources that comprise said emitter; and (ii) a measurement of atleast one packet from a wireless terminal; and a second signal monitorfor making a second plurality of signal measurements, wherein saidsecond plurality is associated with a plurality of signal sources thatcomprise said emitter; wherein said first plurality of signal strengthmeasurements, said second plurality of signal strength measurements,said identifier, and said measurement of said at least one packet areused to determine the location of said wireless terminal.
 17. Theapparatus of claim 16 further comprising a location estimation serverfor: smoothing (i) said first plurality to form a first set of smoothedmeasurements and (ii) said second plurality to form a second set ofsmoothed measurements; interpolating (i) said first set of smoothedmeasurements across two dimensions to form a first scalar array ofsignal samples and (ii) said second set of smoothed measurements acrosstwo dimensions to form a second scalar array of signal samples; andgenerating a vector array of synthetic signal strength vectors based onsaid first scalar array of signal samples and said second scalar arrayof signal samples.
 18. The apparatus of claim 17 wherein said locationestimation server is also for matching a presented signal strengthvector to at least one of said synthetic signal strength vectors. 19.The apparatus of claim 17 wherein smoothing is based on a generalizedadditive model.
 20. The apparatus of claim 17 wherein interpolating saidfirst plurality is performed using Akima splines.
 21. The apparatus ofclaim 17 wherein said location estimation server is also for assigning asignal value in place of a missing signal strength measurement.
 22. Theapparatus of claim 21 wherein said signal value is less than or equal to−92 dBm.
 23. The apparatus of claim 16 wherein the signal strengthmeasurement that represents a signal source is one of (i) the median ofand (ii) the mean of more than one signal strength measurement made overtime of said signal source.
 24. The apparatus of claim 16 wherein anaccess point is collocated with said first signal monitor.
 25. Theapparatus of claim 16 wherein said emitter is stationary.
 26. Theapparatus of claim 16 wherein said identifier indicates the mediumaccess control address of said emitter.
 27. The apparatus of claim 16wherein said identifier indicates the location of said emitter.
 28. Anapparatus comprising: a network interface for acquiring a firstplurality of signal strength measurements that are received by a firstsignal monitor, wherein said first plurality represents a plurality ofsignal sources; and a processor for: (i) smoothing said first pluralityto form a first set; (ii) interpolating said first set to form a firstscalar array of signal samples across two dimensions; and (iii)generating a vector array of synthetic signal strength vectors based onsaid first scalar array of signal samples and a second scalar array ofsignal samples.
 29. The apparatus of claim 28 further comprising: afirst emitter for transmitting a first signal and a first identifier;said first signal monitor for making: (i) said first plurality of signalstrength measurements of said first signal, wherein said first pluralityis associated with a plurality of signal sources that comprise saidfirst emitter; and (ii) a measurement of at least one packet from awireless terminal; and a second signal monitor for making a secondplurality of signal strength measurements, wherein said second pluralityis associated with a plurality of signal sources that comprise saidfirst emitter; wherein said first plurality of signal strengthmeasurements, said second plurality of signal strength measurements,said first identifier, and said measurement of said at least one packetare used to determine the location of said wireless-terminal.
 30. Theapparatus of claim 29 further comprising a second emitter fortransmitting a second signal and a second identifier, wherein saidsecond signal and said second identifier are used to determine thelocation of said wireless terminal.